Recently, research has been ongoing into next-generation multimedia wireless communication systems. As such a system, there is demand for a system that processes and transmits various information such as an image, wireless data, etc. beyond initial voice centered services. One aim of a wireless communication system is to facilitate reliable communication of a plurality of users irrespective of location and mobility. However, a wireless channel undergoes various problems such as path loss, shadowing, fading, noise, limited bandwidth, power limitation of user equipment (UE), and interference between different users. With regard to a design of a wireless communication system, other challenges include resource allocation, mobility issues associated with suddenly changed physical channels, portability, and design for providing security and privacy.
When a transport channel undergoes deep fading, if another version or replica of a transmitted signal is not separately transmitted, a receiver has difficulty in determining the transmitted signal. Resource corresponding to this separate version or replica is called diversity and is one of the most important elements involved in reliable transmission over a radio channel. When the diversity is used, data transmission capacity or data transmission reliability can be maximized. A system for implementing diversity via multiple transmit (Tx) antennas and multiple receive (Rx) antennas is called a multiple input multiple output (MIMO) system.
A scheme for diversity in a MIMO system includes, for example, a space frequency block code (SFBC), a space time block code (STBC), cyclic delay diversity (CDD), frequency switched transmit diversity (FSTD), time switched transmit diversity (TSTD), precoding vector switching (PVS), spatial multiplexing (SM), etc.
One system under consideration in the post third generation system is an orthogonal frequency division multiplexing (OFDM) system that can mitigate an inter-symbol interference (ISI) effect with low complexity. The OFDM system is adapted to transform serial input data symbols into N parallel data symbols and transmit the data symbols with N subcarriers. The subcarriers maintain orthogonality in the frequency domain. Orthogonal frequency division multiple access (OFDMA) refers to multiple access by independently providing each user with some of available subcarriers in a system using the OFDM modulation scheme.
FIG. 1 illustrates a wireless communication system.
Referring to FIG. 1, the wireless communication system includes at least one base station (BS) 20. Each BS 20 provides a communication service to specific geographical areas 20a, 20b, and 20c (each of which generally referred to as a cell). The cell may be re-divided into plural regions (each referred to as a sector). A user equipment (UE) 10 may be fixed or have mobility and may also be referred to as other terms such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, a personal digital assistant (PDA), a wireless modem, a handheld device, etc. In general, the BS 20 refers to a fixed station that communicates with the UE 10 and may also be called an evolved-NodeB (eNB), a base transceiver system (BTS), an access point, etc.
Hereinafter, downlink (DL) refers to communication from a BS to a UE and uplink (UL) refers to communication from a UE to a BS. For DL, a transmitter may be included in a BS and a receiver may be included in a UE. For UL, a transmitter may be included in a UE and a receiver may be included in a BS.
The wireless communication system may be any one of a multiple input multiple output (MIMO) system, a multiple input single output (MISO) system, a single input single output (SISO) system, and a single input multiple output (SIMO) system. The MIMO system uses a plurality of Tx antennas and a plurality of Rx antennas. The MISO system uses a plurality of Tx antennas and one Rx antenna. The SISO system uses one Tx antenna and one Rx antenna. The SIMO system uses one Tx antenna and a plurality of Rx antennas.
Hereinafter, a Tx antenna refers to a physical or logical antenna used to transmit one signal or stream and an Rx antenna refers to a physical or logical antenna used to receive one signal or stream.
A 3rd generation partnership project (3GPP) long term evolution (LTE) system adopts such MIMO. Hereinafter, the LTE system will be described in greater detail.
FIG. 2 illustrates a structure of a radio frame in 3GPP LTE.
Referring to FIG. 2, the radio frame includes 10 subframes each of which includes two slots. Slots in the radio frame are denoted by slot numbers 0 to 19. Time taken to transmit one subframe is referred to as a transmission time interval (TTI). The TTI may be a scheduling unit for data transmission. For example, one radio frame is 10 ms long, one subframe is 1 ms long, and one slot is 0.5 ms long.
The structure of the radio frame is purely exemplary and the number or subframes included in the radio frame or the number of slots included in the subframe may be changed in various ways.
FIG. 3 is a diagram illustrating a resource grid of one UL slot in 3GPP LTE.
Referring to FIG. 3, a UL slot includes a plurality of orthogonal frequency division multiplexing (OFDM) symbols in the time domain and NUL resource blocks (RBs) in the frequency domain. An OFDM symbol is used to represent one symbol period and may be referred to as an SC-FDMA symbol, an OFDMA symbol, or a symbol period according to a system. An RB is a resource allocation unit and includes a plurality of subcarriers in the frequency domain. NUL, the number of RBs included in the UL slot depends upon a UL transmission bandwidth configured in a cell. Each element on the resource grid is referred to as a resource element.
FIG. 3 illustrates an example in which one RB includes 7 OFDM symbols in the time domain and 7×12 resource elements including 12 subcarriers in the frequency domain. However, the number of subcarriers in the RB and the number of OFDM symbols are not limited thereto. The number of subcarriers or the number of OFDM symbols included in the RB may be changed in various ways. The number of OFDM symbols may vary according to a length of cyclic prefix (CP). For example, in the case of a normal CP, the number of OFDM symbols is 7, and in the case of an extended CP, the number of OFDM symbols is 6.
The resource grid of one UL slot in the 3GPP LTE of FIG. 3 can also be applied to a resource grid of a DL slot.
FIG. 4 illustrates a structure of a DL subframe.
The DL subframe includes two slots in the time domain. Each slot includes 7 OFDM symbols in the case of a normal CP. Up to three OFDM symbols (up to four OFDM symbols for a bandwidth of 1.4 MHz) at the start of the first slot in a downlink subframe are used for a control region to which control channels are allocated and the other OFDM symbols of the downlink subframe are used for a data region to which a physical downlink shared channel (PDSCH) is allocated. The PDSCH refers to a channel for transmitting data to a UE from a BS.
A physical downlink control channel (PDCCH) may deliver information about resource allocation (referred to as DL grant) and a transport format for a downlink shared channel (DL-SCH), resource allocation information (referred to as UL grant) about an uplink shared channel (UL-SCH), paging information of a paging channel (PCH), system information on the DL-SCH, information about resource allocation for a higher-layer control message such as a random access response transmitted on the PDSCH, a set of transmission power control (TPC) commands for individual UEs of a random UE group, transmission power control information, voice over Internet protocol (VoIP) activation information, etc. Control information transmitted the aforementioned PDCCH is referred to as DL control information (DCI).
Hereinafter, a DL reference signal (RS) will be described in greater detail.
In a 3GPP LTE system, two types of DL RSs for a unicast service are defined as a common RS, that is, a cell-specific RS (CRS) and a dedicated RS, that is, a UE-specific RS (DRS).
The CRS is an RS shared among all UEs of a cell and is used to acquire information about a channel state and in measuring handover. The DRS is an RS for a specific UE and is used to demodulate data. The CRS may be a cell-specific RS and the DRS may be a UE-specific RS.
A UE measures the CRS and informs a BS of feedback information such as channel quality information (CQI), precoding matrix indicator (PMI), and rank indicator (RI). The BS performs DL frequency domain scheduling using the feedback information received from the UE.
The BS allocates resources in consideration of an amount of radio resources to be allocated to an RS, exclusive locations of the CRS and the DRS, locations of synchronization channel (SCH) and broadcast channel (BCH), density of the DRS, etc. in order to transmit the RS to the UE.
In this case, when a relatively large amount of resources are allocated to the RS, although high channel estimation performance can be achieved, a data transfer rate is relatively reduced. When a relatively small amount of resources are allocated to the RS, although a high data transfer rate can be obtained, density of the RS is reduced, resulting in degraded channel estimation performance. Thus, effective resource allocation to the RS in consideration of channel estimation, data transfer rate, etc. is an important factor in determining system performance.
In a 3GPP LTE system, the DRS is used for data demodulation only and the CRS is used for both channel information acquisition and data demodulation. In particular, the CRS is transmitted every subframe in a wide band and transmitted per antenna port of the BS. For example, when the number of Rx antennas of the BS is two, CRSs are transmitted to antenna ports #0 and #1. When the number of Rx antennas of the BS is four, CRSs are transmitted to antenna ports #0 to #3.
FIG. 5 illustrates an example of a structure of a UL subframe in 3GPP LTE.
Referring to FIG. 5, the UL subframe may be divided into a control region and a data region. A physical uplink control channel (PUCCH) for delivering UL control information is allocated to the control region. A physical uplink shared channel (PUSCH) for delivering UL data is allocated to the data region. To maintain single carrier properties, RBs allocated to one UE are contiguous. One UE cannot simultaneously transmit the PUCCH and the PUSCH.
The PUCCH for one UE is allocated to an RB pair in a subframe. RBs of the RB pair occupy different subcarriers in a first slot and a second slot. A frequency occupied by the RBs of the RB pair allocated to the PUCCH is changed at a slot boundary. As the UE transmits UL control information over time through different subcarriers, frequency diversity gain can be obtained.
The UL control information transmitted on the PUCCH may include hybrid automatic repeat request (HARQ) acknowledgement/negative acknowledgement (ACK/NACK), channel quality indicator (CQI) indicating a DL channel state, scheduling request (SR) as a UL radio resource allocation request, etc.
The PUSCH is mapped to an uplink shared channel (UL-SCH) that is a transport channel. UL data transmitted on the PUSCH may be a transport block that is a data block for UL-SCH transmitted during the TTI. The transport block may be user information. Alternatively, the UL data may be multiplexed data. The multiplexed data may be obtained by multiplexing the control information and transfer block for the UL-SCH. For example, the control information multiplexed to data may include a CQI, a precoding matrix indicator (PMI), HARQ ACK/NACK, a rank indicator (RI), or the like. Alternatively, the UL data may include control information alone.
A high data transfer rate is required. A most basic and stable solution is to increase bandwidth.
However, frequency resources are currently saturated and various technologies have been partially used in wide frequency bands. Thus, as a method for ensuring a wide bandwidths in order to satisfy requirements for higher data transfer rate, scattered bands are designed to satisfy basic requirements for operations of independent systems, and carrier aggregation (CA) that refers to binding a plurality of bands to one system has been introduced. In this case, a band for independent management is defined as a component carrier (CC).
CA technology is adapted in an LTE-Advanced (LTE-A) system as well as by an LTE system.
Carriers Aggregation
A CA system refers to a system that collects one or more carriers having a smaller band than a target wide band to configure a wide band when a wireless communication system supports the wide band. The CA system may be referred to as other terms such as a multiple carrier system, bandwidth aggregation system, etc. The CA system may be categorized into a contiguous CA system with contiguous carriers and a non-contiguous CA system with noncontiguous carriers. Hereinafter, the multiple carrier system or the CA system needs to be understood as both cases in which component carriers are contiguous and noncontiguous.
In the contiguous CA system, a guard band may be present between carriers. When one or more carriers are collected, a target carrier may use a bandwidth used in a conventional system without change for backward compatibility with the conventional system. For example, the 3GPP LTE system supports bandwidths of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz. Alternatively, the 3GPP LTE system may define new bandwidths to configure a wide band instead of the bandwidths of the conventional system.
In a CA system, a UE can simultaneously transmit or receive one or plural carriers according to capacity thereof.
FIG. 6 illustrates an example of communication in a single component carrier situation. The example illustrated in FIG. 6 may correspond to communication in an LTE system.
Referring to FIG. 6, a general wireless communication system using a general frequency division duplex (FDD) scheme transmits and receives data in one DL band and one UL band corresponding thereto. A BS and a UE transmit and receive data and/or control information scheduled in units of subframes. The data is transmitted and received through a data region configured in an UL/DL subframe and the control information is transmitted and received through a control region configured in the UL/DL subframe. To this end, the UL/DL subframe delivers a signal through various physical channels. FIG. 7 is based on the FDD scheme for convenience of description. However, the above description can be applied to a time division duplexing (TDD) scheme by dividing a radio frame into UL and DL in the time domain.
FIG. 7 illustrates an example of communication in a multiple component carrier situation.
The example of FIG. 7 may correspond to communication in an LTE-A system.
The LTE-A system uses carrier aggregation, bandwidth aggregation, or spectrum aggregation technologies to collect a plurality of UL/DL frequency blocks to use wider UL/DL bandwidths in order to use wider frequency bands. Each frequency block is transmitted using a component carrier (CC). Throughout this specification, the CC may refer to a frequency block for CA and a center carrier of the frequency block according to context, which are interchangeably used.
On the other hand, although the 3GPP LTE system supports a case in which DL and UL bandwidths are configured in different ways, one component carrier (CC) is assumed. The 3GPP LTE system may support a maximum of 20 MHz and have different UL and DL bandwidths, but support only one CC in UL and DL.
However, spectrum aggregation (which is also referred to as bandwidth aggregation or carrier aggregation) supports a plurality of CCs. For example, when five CCs are allocated as granularity of a carrier unit having a bandwidth of 20 MHz, a maximum bandwidth of 100 MHz can be supported.
One DL CC or a pair of UL CC and DL CC may correspond to one cell. One cell basically includes one DL CC and optional UL CC. Thus, it is deemed that a UE that communicates with a BS through a plurality of DL CCs receives services from a plurality of serving cells. In this case, DL includes a plurality of DL CCs but UL may use only one CC. Thus, it is deemed that the UE receives services from a plurality of serving cells in DL and receives a service from only one serving cell in UL.
From this point, the serving cell may be categorized into a primary cell and a secondary cell. The primary cell operates at a primary frequency and is a cell configured as a primary cell while a UE perform an initial connection establishment procedure, initiates a connection reestablishment procedure, or performs a handover procedure. The primary cell may also be referred to as a reference cell. The secondary cell may operate at a secondary frequency, may be configured after RRC connection establishment, and may be used to provide additional radio resources. At least one primary cell may always be configured as the primary cell and the secondary cell may be added/modified/released via higher layer signaling (e.g., an RRC message).
Referring to FIG. 7, five CCs of 20 MHz may be collected in each of UL and DU to support a bandwidth of 100 MHz. CCs may be contiguous or noncontiguous in the frequency domain. For convenience of description, FIG. 9 illustrates a case in which a bandwidth of UL CC and a bandwidth of DL CC are the same and symmetrical with each other. However, bandwidths of CCs may be independently determined. For example, the UL CC bandwidth may be configured as 5 MHz (UL CC0)+20 MHz (UL CC1)+20 MHz (UL CC2)+20 MHz (UL CC3)+5 MHz (UL CC4). In addition, asymmetrical CA may be possible such that the number of UL CCs and the number of DL CCs differ. The asymmetrical CA may be generated due to limitation of an available frequency band or intentionally configured via network configuration. For example, even if a total band of a system includes N CCs, a frequency band for reception of a specific UE may be limited to M (<N) CCs. Various parameters for CA may be configured cell-specifically, UE group-specifically, or UE-specifically.
FIG. 7 illustrates an example in which a UL signal and a DL signal are transmitted through CCs that are mapped in one-to-one correspondence. However, a CC for actually transmitting a signal may vary according to network configuration or signal type.
For example, when a scheduling command is transmitted through DL CC1 in DL, data based on the scheduling command may be executed through a different DL CC or UL CC. In addition, control information associated with a DL CC may be transmitted through a specific UL CC in UL irrespective of mapping. Similarly, DL control information may also be transmitted through a specific DL CC.
FIG. 8 is a block diagram for explanation of a single carrier (SC)-TDMA transmission scheme that is a UL access scheme adopted in 3GPP LTE.
SC-FDMA is adapted for UL of LTE. SC-FDMA is similar to orthogonal frequency division multiplexing (OFDM) but reduces a peak to average power ratio (PAPR) to reduce power consumption of a portable terminal and costs of a power amplifier.
SC-FDMA is very similar to OFDM in that signals are also separately transmitted through subcarriers using fast Fourier transform (FFT) and inverse-FFT (IFFT). In addition, SC-FDMA is also the same as conventional OFDM technology in that a simple equalizer in the frequency domain can also be used with respect to inter-symbol interference (ISI) caused by multipath fading by using a guard interval (cyclic prefix). However, SC-FDMA is an additional unique technology in that a PAPR at a receiver is reduced by about 2 to 3 dB to improve power efficiency of a transmitter.
That is, problems arise with regard to a conventional OFDM transmitter in that signals carried in each subcarrier on the frequency axis are converted into signals of the time axis via IFFT. That is, the IFFT is performed by performing the same calculation in parallel, thereby increasing PAPR.
Referring to FIG. 8, as one solution to this problem, SC-FDMA performs discrete Fourier transform (DFT) 102 on information prior to mapping a signal to a subcarrier. The signal spread (or precoded, having the same meaning) via the DFT is mapped 13 to the subcarrier, and then IFFT 14 is performed on the signal to form a signal of the time axis.
In this case, according to a relationship of the DFT 12, the subcarrier mapping 13, and the IFFT 14, SC-FDMA is advantageous in terms of transmit power efficiency in that a PAPR of a signal of the time axis is not dramatically increased after the IFFT 14 unlike OFDM.
That is, a transmission scheme in which IFFT is performed after DFT spreading is referred to as SC-FDMA.
Due to the advantages of SC-FDMA, robustness of a multipath channel can be achieved and simultaneously the disadvantages of the conventional OFDM of increasing PAPR can be basically overcome via IFFT calculation by adopting a similar structure to OFDM, and thus, an effective power amplifier can be used. SC-FDMA may also be called DFT spread OFDM (DFT-s-OFDM) having the same meaning as SC-FDMA.
That is, in SC-FDMA, peak-to-average power ratio (PAPR) or cubic metric (CM) may be reduced. When the SC-FDMA transmission scheme is used, a non-linear distortion period of a power amplifier can be avoided, and thus, transmission power efficiency of a UE with limited power consumption can be increased. Thus, user throughput can be increased.
3GPP has actively conducted into the LTA-A standard as improved LTE. During standardization of LTE-A, SC-FDMA based technologies and OFDM technologies were also competitively discussed as in standardization of LTE, but a clustered-DFT-s-OFDM scheme for allowing non-contiguous resource allocation was adopted.
An LTE-A system will be described in greater detail.
FIG. 9 is a block diagram for explanation of the clustered DFT-s-OFDM transmission scheme adopted as a UL access scheme in the LTE-A standard.
As an important feature of the clustered DFT-s-OFDM scheme, frequency selective resource allocation may be possible so as to flexibly handle frequency selective fading.
In this case, in clustered DFT-s-OFDM adapted as a UL access scheme of LTE-A, non-contiguous resource allocation is allowed unlike SC-FIRMA as a conventional UL access scheme of LTE and thus, transmitted UL data can be divided in various cluster units.
That is, the LTE system maintains single carrier property for UL. On the other hand, the LTE-A system allows a case in which DFT precoded data is noncontiguously allocated on the time axis or a PUSCH and a PUCCH are simultaneously transmitted.